Solution Chemistry Example 5

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Example 5

hard
A student prepares a solution by dissolving 8080 g of KNO3\text{KNO}_3 in 100100 g of water at 60°C60°\text{C} (solubility = 106 g/100 g106\,\text{g/100 g}), then slowly cools it to 20°C20°\text{C} (solubility = 31 g/100 g31\,\text{g/100 g}). How many grams of KNO3\text{KNO}_3 will crystallize out?

Solution

  1. 1
    At 60°C60°\text{C}: solubility is 106 g/100 g water106\,\text{g/100 g water}. With 8080 g dissolved, the solution is unsaturated.
  2. 2
    At 20°C20°\text{C}: solubility drops to 31 g/100 g water31\,\text{g/100 g water}. Only 3131 g can remain dissolved.
  3. 3
    Mass that crystallizes = 80−31=4980 - 31 = 49 g of KNO3\text{KNO}_3.

Answer

49 g of KNO3 crystallizes out49\,\text{g of KNO}_3\text{ crystallizes out}
This is the basis of purification by recrystallization — dissolve a substance in hot solvent where solubility is high, then cool to precipitate pure crystals. The temperature dependence of solubility makes this an effective technique.

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A homogeneous mixture formed when one or more solutes are completely dissolved in a solvent at the molecular level, resulting in a uniform composition throughout.

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